Push-pull chromophores have attracted enormous interests in the scientific community because of their interesting photophysical, photosensitizing and non-linear optical properties [Kanis et al., Chem. Rev. 1994, 94, 195; Long et al., Angew. Chem., Int. Ed. Engl. 1995, 34, 21]. In recent decades, this kind of compounds has drawn a lot of attention in material science due to their capability to function as photosensitizers and optoelectronic materials with facile modification of the HOMO and LUMO energy levels and tunable self-assembly morphology. A typical organic push-pull chromophore consists of an electron donor and an electron acceptor connected by a π-conjugated spacer [Grimsdale et al., Chem. Rev. 2009, 109, 897; Liang et al., Chem. Soc. Rev. 2013, 42, 3453]. The HOMO mainly lies on the electron donor moiety, while the electron acceptor moiety contributes to the LUMO. By the judicious design and suitable choice of the donor and acceptor pair, desirable photophysical and chemical properties can be achieved and widely applied in organic photovoltaics (OPVs) and organic light-emitting diodes (OLEDs). For OPVs, strong electron acceptors have been employed to lower the LUMO level, while moderate strengths of electron donors have been selected to adjust the HOMO level. Also, the electronic absorption profile could be tuned by the selection of donor-acceptor strengths so that the materials could have a better coverage for the absorption of the solar spectrum [Cheng et al., Chem. Rev. 2009, 109, 5868; Kippelen et al., Energy Environ. Sci., 2009, 2, 251; Liang et al., Acc. Chem. Res. 2010, 43, 1227; Li et al., Nat. Photonics 2012, 6, 153; Chen et al., Acc. Chem. Res. 2013, 46, 2645]. On the other hand, in order to develop OLED materials with desirable emission colors, with clever selection of the donor-acceptor couple, unnecessary optimization by trial-and-error and tedious synthesis could be avoided [Wang et al., Chem. Soc. Rev. 2010, 39, 2387; Uoyama et al., Nature, 2012, 492, 234; Li et al., Chem. Soc. Rev. 2013, 42, 8416; Xu et al., Chem Soc. Rev. 2014, 43, 3259].
To date, new functional materials with main group elements have drawn much attention in the past decade [Hissler et al., Coord. Chem. Rev. 2003, 244, 1], especially those main group compounds incorporated with π-conjugated framework. Among those main group elements, group 13 boron has received immense attention due to three major characteristic features. The first one is the fact that boron can form three or four covalent bonds, adopting trigonal planar or tetrahedral geometry, respectively. This geometrical feature may be used as a building block for constructing complex molecules. Secondly, three-coordinated boron(III) compounds consist of a vacant p-orbital, which can form pπ-π* conjugation effectively when connected to the π-conjugated system [Entwistle et al., Angew. Chem. Int. Ed. 2002, 41, 2927; Hudson et al., Acc. Chem. Res. 2009, 42, 1584; Jäkle, F. Chem. Rev. 2010, 110, 3985]. The electronic and photophysical properties can then be facilely tuned by the π-conjugated system. Thirdly, three-coordinated boron is well-known as a quintessential Lewis acid. The empty pπ-orbital of the boron center can readily form unique complexes with Lewis bases by nucleophilic attack [Jäkle, F. Coord. Chem. Rev. 2006, 250, 1107; Hudnall et al., Acc. Chem. Res. 2009, 42, 388; Wase et al., Chem. Rev. 2010, 110, 3958]. Moreover, the intrinsic electron-withdrawing ability of boron(III) can induce excited state anisotropism. By exploiting these features, researchers have applied boron-containing compounds to various applications. Despite the multi-functionality of boron(III) atom, this class of compounds has been less explored in different π-conjugated systems because of the dominating and extensive studies on the family of boron dipyrromethenes (BODIPY) [Loudet et al., Chem. Rev. 2007, 107, 4891; Kamkaew et al., Chem. Soc. Rev. 2013, 42, 77; Bessette et al., Chem. Soc. Rev. 2014, 43, 3342] and boron subphthalocyanines (SubPc) [Claessens et al., Chem. Rev. 2002, 102, 835; Heremans et al., Acc. Chem. Res. 2009, 42, 1740; Bottari et al., Chem. Rev. 2010, 110, 6768; Claessens, C. G.; Gonzalez-Rodriguez et al., Chem. Rev. 2014, 114, 2192]. Besides these two classes of boron(III) compounds, it is envisaged that the utilization of the intrinsic electron-withdrawing properties of the boron(III) atom in donor-acceptor systems with precise molecular engineering would readily give rise to interesting materials for various applications.
Donor-acceptor systems have been scarcely reported to exhibit gold-like metallic lusters in their crystals [Anex et al., Chem. Phys. 1976, 12, 89; Li et al., J. Am. Chem. Soc. 1998, 120, 2206; Parakka et al., Synth. Met. 1995, 68, 275; Ogura et al., Org. Biomol. Chem. 2003, 1, 3845; Evans et al., Org. Biomol. Chem. 2013, 11, 3871]. There is still no precise conclusion on the origin of the metallic luster behavior in organic compounds. However, it has been proposed that such highly reflective crystals are originated from the presence of both donor-acceptor systems and strong π-π stacking character. By utilizing the metallic luster of these organic compounds, it is believed that a new class of optical reflectors can be achieved. Besides metallic mirrors, Bragg reflectors (dielectric mirrors) and photonic crystals for use as optical reflectors or filters have been studied extensively. However, Bragg reflectors require multi-layer preparation, where the layers are composed of materials of different refractive indices [Holtz et al., Nature 1997, 389, 829; Fink et al., Science, 1998, 282, 1679; Weber et al., Science 2000, 287, 2451; Yetisen et al., Chem. Rev. 2014, 114, 10654], while 3D architecture is essential for optical reflectors based on photonic crystals [Edrington et al., Adv. Mater 2001, 13, 421; Ge, J.; Yin, Y., Angew. Chem. Int. Ed. 2011, 50, 1492; Gonzalez-Urbina et al., Chem. Rev. 2012, 112, 2268; Freymann et al., Chem. Soc. Rev. 2013, 42, 2528]. Unlike them, only spin-coating or vacuum deposition is required for optical reflectors that are composed of organic compounds with metallic luster behavior.
On the other hand, electroactive organic materials have been suggested as good alternatives of traditional Si, Ge and GaAs semiconductors that have to face the problem of scaling-down in cell size [Raymo, F. M. Adv. Mater 2002, 14, 401]. In contrast to memory devices based on the traditional inorganic semiconductors, organic compounds have the advantages of being capable of fabricating devices that are low cost, large scalability and data storage capacity, good processability, flexible and light weight. The devices can be fabricated by forming a composite active layer and switch with two electrodes. Unlike inorganic memories which are based on the amount of charges stored in the devices, the memory effect in organic memory devices is based on the electrical bistability of conductance (resistance), where a low-conductance (OFF) state switches to a high-conductance (ON) state [Yang et al., Adv. Funct. Mater. 2006, 16, 1001; Scott, J. C.; Bozano, L. D.; Adv. Mater 2007, 19, 1452; Heremans et al., Chem. Mater. 2011, 23, 341]. To achieve electrical bistability in organic systems, donor-acceptor couples have to be employed and they have been demonstrated as one of the successful approaches involving electric-field-induced charge transfer between conjugated compounds.